High brightness lighting devices (e.g. lighting devices able to output relatively high lumen density) are used in various applications, such as in automotive head lights and for fluid (such as liquid or gas) purification. Traditional high brightness lighting devices based on incandescent and gas-discharge technologies are currently being replaced by more energy efficient solid state based alternatives, such as light emitting diode (LED) based lighting devices. US 2010/0176067 shows an example of an LED based photocatalytic reactor for removing contaminants from water. It is a challenge in design of solid state based lighting devices to provide high lumen density, as the lumen output per solid state based light source is limited.